The type attribute is the hint to make sure the catalog dictionary is treated special, and its entries are added to the PDF Document Catalog. The same extension can easily be used for other dictionaries, like ViewerPreferences. Using the same terminology as in the PDF Reference seems to make sense, as it's a PDF extension. Using the exact same possible values could lead to an easier translation to PDF.

Thinking about it some more, and starting to prototype this idea, the entry object seems to be the key, as it is conceivable to also insert a dictionary by specifying it entirely as a PDF expression. Makes me wonder to what extent we should validate the contents... Ultimate flexibility vs. the risk of producing corrupt PDF in case of the slightest glitch/typo (?)